Abstract: The heat insulator includes a lower sheet (14) that is larger is both length and width than the upper sheet (12) so that the heat insulator can span over objects such as purlins, rafters and obstructions in a roof structure while maintaining the air gap (20) between the sheets. The air gap protects the reflectivity of the inner surfaces of the sheets, thereby preserving the ability of the sheets to reflect radiant heat. The larger, lower sheet can be formed in a single catenaries from edge to edge of the sheet, or in a multiple array of catenaries to form parallel, tubular air gaps (26), if desired. The heat insulator can be mounted to fibrous insulation (22) or to other more rigid structures, if desired, when placed in a roof structure.